1. Field of the Invention
The invention relates to ultrasonic generators and, in particular, to improve regulation of a generator for driving an ultrasonic transducer used in welding.
2. Description of the Prior Art
Ultrasonic generators for operating transducer/horn assemblies for various ultrasonic applications, such as the welding of plastic parts or the like, are well known. One such generator, which uses pulse-width modulation to control the power output level, is disclosed in U.S. Pat. No. 4,277,710, and is a high-power generator including a transistor bridge inverter circuit connected to a DC source for producing an alternating output current. The ultrasonic power output from that generator can be varied by varying the conduction pulse width of the power conversion circuit, which varies the output voltage to the transducer, thereby varying the ultrasonic amplitude of the transducer stack. Initially, the power (pulse width) of that generator was controlled with a front panel control knob. However, Dukane Corporation, the assignee of U.S. Pat. No. 4,277,710 has also produced a user interface, in the nature of a remote power control option board, that can control the ultrasonic power output remotely through an isolated 4-20 ma current loop signal, which allows users to adjust the power output (pulse width) to suit their processing needs. Typically, the remote power control option is used to control a continuous ultrasonic process, and usually utilizes an ultrasonic wattmeter or a separate energy module to monitor the power level of the ultrasonic process.
Particularly in applications where a number of ultrasonic generating units are used, under common control of a programmable logic controller (PLC), the PLC has been used to monitor the power output signal and to control the 4-20 ma current loop signal to the remote power control board input to regulate the generator pulse width to the desired power output level, in an arrangement similar to that illustrated in FIG. 1. While this method works well for certain ultrasonic processes, the reaction time to process variations is relatively slow, with response times of hundreds to thousands of milliseconds to correct for errors. Furthermore, the external PLC is an expensive item and, while it is necessary in the control of multiple ultrasonic systems, it adds unnecessarily to the cost and complexity of the power control of any one system. Furthermore, special system programming is needed for such PLCs and, if the programming is flawed, the regulation level may contain uncontrolled spurious oscillations.